1. Field of Invention
This invention relates to lighting appliances for dental and medical clinics and in particular to such devices which utilize the combination of an optical zoom mechanism, a heat absorbing filter, and a controllable iris to provide a truly cold variable intensity, sharp focused light beam onto the projected objective area.
2. Brief Summary of Prior Art
The dental operating lights can be generally divided into two groups: (1) those using a light source at one focus of a dichroic-film coated ellipsoidal glass reflector whereupon the reflected light beam is loosely and openly converged at the other focal point, forming an irregular-shaped illuminated area; and (2) those using a light source at one focus of an ellipsoidal metallic reflector that projects at the other focus on an aperture having an opening defining the extent of the beam, and using a heat absorbing filter to eliminate the infrared energy from the lamp. The former group provides a high intensity light on the patient's face in an uncontrollable irregular pattern with most of the infrared portion of the radiation energy produced by the light source resulting in undesirable heating the face of the patient as well as the arms, shoulder, and head of the dentist or doctor. In addition, the color temperature of the illuminating light will change continuously due to the oxidation of the dichroic coating produced by the high intensity heat energy of the light source and the moisture from the air. The latter group generally use a vertically oriented cylindrical bulb mounted in either a bayonet or threaded socket protruding from the end of the ellipsoidal reflector to the nearest conjugate focus. Since the radiation pattern of the vertically oriented cylindrical bulb is inherently circular, conversion of the circular pattern to the desired horizontally oriented pattern will either induce some deficiency in the horizontal dimension of the illuminated area, or sacrifice radiation energy in the vertical direction. In addition, the socket and the base of the light bulb will quite often fuse together after a certain length of service. Consequently, lamp burnout requires costly system replacement instead of the simple replacement of the light bulb. Furthermore, in those lamps that utilize a moving light source relative to a fixed aperture, there is no real advantage to varying the beam intensity since most of its illuminated pattern will be fuzzy and dim except at one single position where the converged light beams are focused right at the light gate.
Moreover, most of these operating lights are subject to the problem of A. C. switching surge (or arcing) which can produce a fire hazard throughout the electrical supply circuit when its on-off switch is used for a certain length of time. In addition, each time the lamp is turned on or off, a "cold-shock" effect acts on the filament of the light bulb, severely limiting the actual service life of the light bulb.